immersion heater controller

Not my field - but, fwiw, this is how I would do it:

Choose one with very little hysteresis. It is going to be switching very frequently - so it will need to be a solid state switch and not a mechanical one.

2kW and 25 gallons will roughly heat that much water by two degree a minute. So you will need mechanical agitation as well as convection, assuming a standard tubular heater. Otherwise it will be nigh on impossible to prevent large over and under shoots from that narrow range, using convection currents alone.

So, I would go for a small bead thermistor-type sensor coupled to an adjustable hysteresis solid state controller, plus a motor driven stirrer. Initially set the hysteresis to around a quarter of the allowable differential. I would go for a very small, low thermal mass, heater. OK, that will mean that its temperature will be well above the set temperature when heating, but it will transfer very little heat to the water once the desired temperature is reached, thus reducing overshoot.

Omega is trhe company that specializes in this sort of thing but their thermistor sensors are not recomenced any higher than 75c with a max of 90c. You are above that so you are probably looking at a thermocouple sensor. The low tech solution is a regular water heater thermostat but I doubt they will give you the accuracy you want.

water will not get hotter than 100 deg at sea level.

Wincey.

Try telling that to the marines! Or, at least, the designers of marine steam turbines and boilers... ;)

We have different water in the US, it boils at 212 degrees. It is only that commie metric water that boils at 100 ;-)

That depends which side of the pond you live on! In the US, 212F is boiling.

Ben Miller

The boiling point of water at 3500 feet above sea level is 205.7°F.

So be careful or you will boil the water due to over temperature or altitude.

What is the end application of the system?.

BillB

Go here:

They have a selector guide to pick the controller. I would recommend also getting a probe type thermocouple to measure the temperature.

I would recommend a solid state relay to run the heater. If you select "DC pulse type" output, the controller will output 10V DC to feed into a external solid state relay (SSR). You will also need to get a SSR with enough current to handle the heater.

You want a PID controller, not a thermostat. PID controllers cycle the heater fast with a variable duty cycle. The duty cycle is adjusted to maintain the set temperature. This results in far finer temperature control because a regular thermostat waits for the temperature to rise and fall. There is a long delay, and thermostats will overshoot. A PID controller cycles the heater on and off without a significant change in temperature. You MUST UNDERSTAND that it will start cycling the heater on and off well above and below the setpoint unlike a thermostat. This is to prevent overshoot. Many people will look at it and say it's broken because it's turning the heater on when it's too hot or it's turning the heater off when it's too cool. In reality it's anticipating that the setpoint is about to be reached and starting to dial up/down the heater. A thermostat will dumbly hold the heater on right up to the setpoint, then of course it will overshoot far more.

I have had good luck with the autotune feature of the Omron controllers in similar applications. Is the immersion heater similar to a domestic water heater element? They don't have too much lag. A element with a lot of thermal mass can be more difficult to control.

I'm sure he means 200F (do-able) not 200C (requires a pressure vessel). M Walter